I discovered the easiest way in the world to win 1v3 insane / very hard comps (post 1.1 patch) to get achievements. UPDATE: Still works as of April 22, 2011. I think it took only 14 minutes and little skill. I made this video to help you get your outmatched achievements if you have been struggling with them.
JUST GET TO 15,000 MINERALS BEFORE COMPS AND YOU WIN.
Pick map: Burning Tide, custom settings
Pick Protoss vs. 3 Zergs (Zergs seem to waste a lot of mins)
Start building probes
Build a pylon
Never forget to chronoboost. The whole reason I picked Protoss is for chronoboost so don't forget to constantly chorono.
Dont get supply blocked, you will only need 6 pylons TOTAL
Build 21 probes at each base
The computer will never attack you.
Last but not least, run a probe around the map to pick up +100 mineral boosts. UPDATE: these mineral boosts should respawn after each raise and lowering of the lava tide! Keep picking them up after each tide, this could make the difference between winning and losing!
VIDEO IS VS. 3 INSANE ai ZERG.
THis works for 1v1, 1v2, and 1v3 medium, hard, very hard, and insane. Enjoy all the free easy acheivement points. That's all the outmatched achievements with 0 skill required!
For tips on 1v4 very hard, see my other thread named "Get EVERY SINGLE outmatched achievement w/o skill."